A question about mergeinfo representation versus mergeinfo canonical
meaning after taking account of inheritance etc. (Bear in mind that I'm
not well aquainted with this merging stuff.)

The new function svn_repos_mergeinfo_changed() looks like a useful
helper for things that want to know in what way mergeinfo changed in a
particular revision.

> /* Set @a *deleted_mergeinfo_catalog and @a *added_mergeinfo_catalog
> * to mergeinfo catalogs describing how mergeinfo values on paths
> * (which are the keys of those catalogs) were changed via the commit
> * of @a rev in @a repos. Allocate returned data from @a pool.
> *
> * Every path whose mergeinfo was created or modified in @a rev in
> * such a way that it differs from its value in @a rev - 1 is
> * represented in both returned catalog hashes, and only those paths
> * are represented therein.
> *
> * @since New in 1.6.
> */
> svn_error_t *
> svn_repos_mergeinfo_changed(svn_mergeinfo_catalog_t *deleted_mergeinfo_catalog,
> svn_mergeinfo_catalog_t *added_mergeinfo_catalog,
> svn_repos_t *repos,
> svn_revnum_t rev,
> apr_pool_t *pool);

>From the fact that it reports its results in "svn_mergeinfo_catalog_t"
which has separate representations for "no mergeinfo" and "empty
mergeinfo", it looks like it reports changes to the mergeinfo
representation, so, for example, if somebody "cleans up" their mergeinfo
properties by performing some elision of mergeinfo that would be
inherited, or by removing inoperative revision ranges, that would be
reported as a change.

Have I understood it correctly?

There's nothing wrong with that as such, but I'm wondering:

* Do we have the complementary support function(s) for determining
whether the canonical meaning of the mergeinfo has changed?

* Where are we drawing the architectural line between APIs that deal
with the representation and APIs that deal with the canonical meaning?

* Do we need to make that distinction clearer in the doc strings,
perhaps using terminology like "mergeinfo properties" versus "canonical
mergeinfo"?

Also, does this new function need "authz_read" functionality like the
v1.5 function svn_repos_fs_get_mergeinfo() has, or need to document its
behaviour in this regard?